For digital data communication, the functions necessary for communication are generally partitioned in a layered architecture. Layer 1, the physical layer, describes the electrical or optical signaling, mechanical, and timing characteristics of a data link. Layer 2, the data link layer, determines how signals transmitted and received by the physical layer should be interpreted; generally, the data link layer provides framing, and may also include authentication, network layer address negotiation, loopback detection, and multiplexing of higher-layer protocols. Layer 3, the network layer, is responsible for end-to-end addressing and routing of packets traversing a network (or collection of networks) generally consisting of many separate data links. Four additional higher layers can provide additional high-level functionality, including packet ordering, ensuring reliable transmission, error correction, flow control, etc.
The Point-to-Point Protocol (PPP) is a data link layer protocol. It is described in Internet Engineering Task Force (IETF) Request For Comments (RFC) 1661, “The Point-to-Point Protocol (PPP)”, (1994). As described in RFC 1661, PPP provides a method for encapsulating multi-protocol datagrams, a Link Control Protocol (LCP) for establishing, configuring, and testing a data link, and a family of Network Control Protocols (NCPs) for establishing and configuring different network-layer protocols.
PPP was initially envisioned for use with low-speed services, including modem connections using analog phone lines. It was found, however, that PPP served a wide variety of connection types, including high-speed lines. For instance, PPP is now deployed for use with SONET (Synchronous Optical Network) physical layer equipment, in what is known as PoS (Packet over SONET). PoS is described in IETF RFC 2615, “PPP over SONET/SDH”, (1999), using HDLC (High-level Data Link Control)-like framing as described in IETF RFC 1662, “PPP in HDLC-like Framing”, (1994). SONET physical links use an optical carrier with one of several defined data signaling speeds. For instance, OC-1, the slowest SONET format, signals at a rate of 51.84 Mbps (million bits-per-second). OC-12 is twelve times as fast, at 622.08 Mbps, and OC-192 is 192 times as fast, at 9,953.28 Mbps, almost ten billion bits per second.